The tryptophan phosphorescence of porcine and mutant bovine odorant-binding proteins: a probe for the local protein structure and dynamics

Vertebrate odorant-binding proteins (OBPs) are small extracellular proteins belonging to the lipocalin superfamily. They have been supposed to play a role in events of odorant molecules detection by carrying, deactivating, and/or selecting odorant molecules. The OBPs share a conserved folding pattern, an eight-stranded beta-barrel flanked by an alpha-helix at the C-terminal end of the polypeptide chain. The beta-barrel creates a central nonpolar cavity whose role is to bind and transport hydrophobic odorant molecules. These proteins reversibly bind odorant molecules with dissociation constants ranging from nanomolar to micromolar range. In this work, we have studied the structural features of the OBP from pig and from cow by phosphorescence spectroscopy. The obtained results demonstrate that the indolic phosphorescence of the two studied proteins can be readily detected at ambient temperature solutions and that it is owed exclusively to the internal tryptophan residue located next to the ligand binding cavity, which is generally conserved in the mammalian OBPs. In addition, while both the phosphorescence spectrum and the lifetime yield a picture of the fold of the studied protein in good agreement with the protein crystallographic structures, the triplet probe points out that in solution the polypeptide structure of the both investigated OBPs exists as a multiplicity of slowly interconverting protein conformations. Finally, this work also demonstrates that it is possible to directly detect the binding of the ligands to OBPs as variations of the protein luminescence features, thus, representing the very first observation reported in the literature so far that a fast and direct assay can be used for monitoring the binding of ligands to OBPs.